The invention relates to a device for interrogating and correcting a serial data signal. A data signal has the property that nominally the signal has one of a finite set of values at any instant. The complete set may comprise, for example, 2, 3 or 4 elements. In reality other signal values can also occur due to disturbances or finite edge steepnesses. During interrogation, discrimination also takes place and each interrogation value is translated into an element of said set. Thus, a series of interrogation data is formed, for example, interrogation bits. Said disturbances and finite edge steepnesses can cause incorrect elements to occur in the series of interrogation data. In some cases these incorrect elements can be corrected. For example, additional data may be added to the data signal, for example by the inclusion of additional data cells, so that an error correcting code is obtained. The addition of such additional data cells reduces the efficiency of the channel, which is measured as the quantity of data per unit of time (without taking into account said additional data). A data cell is to be understood to mean herein the smallest logic unit of a data signal, that is to say the smallest time interval from which the data can be recovered in principle without it being necessary to know the data in one or more preceding or subsequent data cells. In the case of bivalent data, a data cell contains at least one data bit. In the biphase code to be described hereinafter, a data cell is identical to the bit cell.
Additional data can be added to the signal for other purposes, for example for making the signal self-clocking. U.S. Pat. No. 3,893,171 describes a device for interrogating and correcting a data signal which is composed in accordance with the MFM (modified frequency modulation) method. This signal has two nominal values and embodies a series of data bits. The signal is interrogated twice per bit cell and each interrogation produces an interrogation bit. The series of nominal interrogation bits consists of series of successively one, two or three "zeroes", each time separated by a single "one". According to said Patent Specification, use is made of the fact that the MFM method introduces a given redundancy. It is thus possible to indicate a single combination of interrogation bits, i.e. two directly successive "1" signals, as being incorrect. If these two "1" signals are flanked with mutually unequal series of "zeroes", unambiguous correction is possible. The inventor of the present invention has found that the possibility of error correction can be enhanced: for a single code, several kinds of error can be corrected and, moreover, the method can be used for different codes.